1. Field of the Invention
The present invention relates to a bonding apparatus and more particularly to a bonding apparatus provided with an ultrasonic bonding horn used in manufacturing of, for example, semiconductor devices.
2. Prior Art
One type of conventional bonding apparatus, a nailhead heat-and-pressure bonding type wire bonding apparatus, is shown in FIG. 5.
In this wire bonding apparatus, a supporting shaft 2 is fastened to a bonding arm 1 and is supported on a bonding head (not shown) either directly or via a lifter arm in a rotatable fashion. The horn support 4 of an ultrasonic horn 3 is mounted to the bonding arm 1. The ultrasonic horn 3 includes a horn body 6 which has a capillary 5 at one end and a vibrator 7 at another end. The vibrator 7 is screw-connected to the horn body 6. A bonding wire (not shown) passes through the capillary 5.
More specifically, the vibrator 7 includes a vibration-generating source 8 that is secured by screws. This screw installation is called a "Langevin" method. The vibrator 7 includes: a horn attachment 9 which is screw-connected to the horn body 6, a vibration-generating source attachment shaft 10 which has a threaded portion formed on both ends and is screw-connected to the horn attachment 9, an insulating pipe 11 which is fitted over the vibration-generating source attachment shaft 10, a vibration-generating source 8 which is obtained from a plurality of stacked doughnut-shaped electrostrictive strain elements or magnetostrictors that are fitted over the insulating pipe 11, and a nut 12 which is screw-connected to the vibration-generating source attachment shaft 10 so that the vibration-generating source 8 is tightened and secured between the nut 12 and the horn attachment 9.
In this conventional ultrasonic horn 3, the vibration-generating source 8 is located on the opposite side of the horn support 4 from the capillary 5. In other words, the vibration-generating source 8 is provided across from the capillary 5. The frequency of the vibrator 7 is adjusted to a desired level by the horn attachment 9 and the nut 12. The acoustic length of the vibrator 7 needs to be an integral multiple of 1/2 of the wavelength. Since there is no reason to use a lengthy vibrator, the vibrator 7 of the length equal to 1/2 of the wavelength is utilized. Furthermore, the free end 13 of the vibrator 7 acts as a vibrational antinode, and the horn attachment portion 14 of the horn attachment 9 acts as a vibrational antinode. Thus, the attachment of the vibrator 7 and the horn attachment 9 is facilitated.
In use, the vibration of the vibration-generating source 8 is transmitted throughout the entire ultrasonic horn 3 so that a standing-wave vibration is created in the ultrasonic horn 3, and the required energy is supplied to the capillary 5. In a non-loaded state (i.e., when bonding is not being performed), the energy accumulates in a stable manner. In addition, the horn 3 (particularly a skillfully crafted ultrasonic horn) is designed dimension-wise so that a node is formed in the horn support 4. Thus, the amount of movement of the horn support 4 is small and the loss of the movement is small even though the ultrasonic horn 3 is mounted to the bonding arm 1. In this non-loaded state, the ultrasonic horn 3 acts like a tuning fork, and the horn support 4 receives vibrations in a symmetrical manner from the left and right sides so that there is no movement to the left or right. The vibration-generating source 8 is ordinarily driven by constant-current driving, etc. so that the amplitude is kept at a prescribed value. When the energy is used for bonding via the capillary 5, the energy equilibrium between the vibration-generating source side and the capillary side loses its balance, and the node of vibration moves, and the energy required for equilibrium is fed in. An ultrasonic bonding is thus performed.
In the conventional ultrasonic horn 3, the vibration-generating source 8 is provided on the opposite side of the horn support 4 from the capillary 5. Accordingly, the energy of the vibration-generating source 8 passes through the horn support 4. Thus, when the vibration energy is consumed by bonding on the capillary side, the energy passing through the horn support 4 is subject to the influence of energy loss which fluctuates greatly according to the structure of the horn support 4, and the energy becomes unstable. The energy loss of the horn support 4 tends to fluctuate greatly when the amount of energy passing through the horn support 4 is large. Accordingly, it is difficult to ascertain the actual loaded conditions from the characteristics of the ultrasonic horn when it is in a non-loaded (or non-bonding) state.
The facts will be described below to a greater extent.
In the non-loaded state (or bonding is not being performed), energy from both sides of the horn support 4 causes distortion with the center of the horn support 4 forming a vibrational node; as a result, the horn support 4 does not move in the axial direction of the ultrasonic horn 3. The reason that the horn support 4 is selected to act as a supporting point is that a minimal energy loss is expected here because of a lack of minimal mechanical vibration. A well-crafted ultrasonic horn is stable in a non-loaded state and shows little loss. During the bonding operation, however, energy is consumed on the capillary side; as a result, the energy balance on both sides of the horn support 4 is destroyed, and vibration occurs in the horn support 4. Consequently, frictional movement occurs between the bonding arm 1 and the horn support 4, so that the relationship between the energy applied to the vibrator 7 and the energy used in the assembly of semiconductor devices is destroyed, lowering the bonding quality.
The energy consumed by the fixed parts of the horn support 4 and capillary 5 cannot be distinguished from the energy consumed in the actual bonding part. Accordingly, bonding under optimal conditions can only be accomplished by adjusting the current applied to the vibration-generating source 8 and the time of current application while examining the actual bonding results. However, the energy which is used needlessly for this purpose is unstable, and the energy used for bonding is also unstable.
In the conventional ultrasonic horn 3, when energy loss occurs during bonding so that the vibrational energy becomes insufficient on the capillary side of the horn support 4, the left-right equilibrium of the horn support 4 is destroyed, and the horn support 4, which is originally supposed to act as a vibrational node, cannot remain static and instead begins to move. As a result, the ultrasonic energy applied is converted into thermal energy between the horn support 4 and the bonding arm 1 or escapes into the bonding arm 1 and the parts attached to the bonding arm 1, thus causing the bonding arm 1, etc. to vibrate. All energy which is accumulated, consumed or released due to such unexpected causes leads to a deterioration in the bonding quality.
Another factor that causes deterioration of the bonding quality is variations in the frequency and impedance of the vibrator. The vibrator 7 shows a frequency variation of several hundred hertz between its minimal-amplitude and its maximum-amplitude. In the conventional structure, the vibration-generating source 8 is installed on one side of the horn support 4; accordingly, the frequency tends to be shifted on the two sides of the horn support 4. Even if there are two frequencies, vibration is actually performed at one frequency or the other. Accordingly, the amplitude of the capillary 5 varies greatly between cases where the vibration occurs at the frequency on the capillary side and cases where the vibration occurs at the frequency on the vibrator side. Furthermore, during actual bonding, the frequency may change suddenly according to fluctuations in the conditions, thus leading to defective bonding.